This invention pertains to a process for the sulfonation of aromatic phosphines, more particularly, the monosulfonation of aromatic phosphines to their corresponding monosulfonated derivatives.
Sulfonated aromatic phosphines find utility as ligands in transition metal-ligand complex catalysts that are useful, for example, in carbonylation processes, such as, hydroformylation processes that convert olefins to aldehydes. More particularly, aromatic phosphine monosulfonated metal salts are useful ligands in rhodium-organophosphine ligand complex catalysts, which are useful in hydroformylation processes to convert unsaturated fatty acids and fatty acid esters to their corresponding fatty acid and fatty acid ester aldehydes. These fatty acid aldehydes and fatty acid ester aldehydes can be further derivatized to the corresponding difunctionalized fatty acid and fatty acid ester alcohols, acids, or amines, which are useful intermediates in the preparation of thermoset polymers and epoxy resins.
Dialkyl monoaryl phosphine monosulfonate metal salts are particularly advantageous in that they tend to exhibit a high degree of activity in hydroformylation processes. Moreover, catalysts prepared with dialkyl monoaryl phosphine monosulfonate metal salts tend to exhibit less sulfonate and aryl group scrambling, thereby resulting in better catalyst selectivity and a higher degree of catalyst stability, as compared with catalysts containing sulfonated triarylphosphine ligand.
The syntheses of aromatic phosphine monosulfonates, disulfonates, or trisulfonates have been described in the art, as indicated for example by the following references: U.S. Pat. Nos. 5,451,698, 5,663,426, 5,684,181, 6,610,881 B1, 6,613,939 B2, and 6,864,387 B1; Bartik, et al., Inorganic Chemistry, vol. 31, 1992, pp. 2667-2670; H. Gulyás, et al., European Journal of Organic Chemistry, 2003, pp. 2775-2781; B. Fell, et al., Journal für praktische Chemie, Chemiker-Zeitung, vol. 336, 1994, pp. 591-595; and S. Hida, et al., Journal of Coordination Chemistry, vol. 41, 1998, pp. 345-348. The foregoing publications mention various problems attendant to sulfonation and work-up of the reaction mixture, including the formation of mixtures of mono-, di-, and trisulfonated products that are difficult to separate; the obtention of low yields of the desired sulfonated product; and the formation and presence in the product of impurity phosphine oxides, sulfites, and most particularly, metal sulfates. The cited publications describe complex work-up procedures of the reaction mixture involving a multitude of extraction steps to reach a purified form of the desired aromatic phosphine sulfonated product. Disadvantageously, the complexity of such work-up procedures makes commercialization of these syntheses cost prohibitive. Consequently, it would be desirable to discover an improved synthesis of a sulfonated aromatic phosphine, preferably, an aromatic phosphine monosulfonate metal salt, that avoids the recovery, impurity, and separation problems of prior art syntheses and that produces in only a few cost-effective steps a purified form of the desired monosulfonated product. It would also be desirable to recover the aromatic phosphine monosulfonate metal salt in high yield.